1. Field of the Invention
The invention relates to the field of electrical over-current protection, and in particular concerns a resistance device to be placed in series with a circuit breaker and load, for limiting the current passed by the breaker at the onset of a fault such as a short circuit.
2. Prior Art
Devices for protecting electrical circuits and loads from current fault conditions are often associated with controllable contactors that couple the load to the power line. In AC motor applications, the contactor arrangement is generally called a motor starter. In a general power distribution network, such contactors are usually called circuit breakers. In a multi-phase circuit, the contactor may have a plurality of associated contact sets, typically operated electromagnetically to connect and disconnect the load and the power line.
In addition to on/off controls for coupling and decoupling the load and the line, a sensing circuit typically monitors the current passing through the contacts from the line to the load. The sensing circuit produces a trip signal that triggers decoupling of the load from the line in the event of a detected fault condition.
Circuit protective devices may respond to fault conditions such as short term excess current due to a short circuit, long term excess current indicating overloading, a ground fault, phase imbalance or the like. Circuits for detecting such faults typically comprise some form of threshold responsive element generating a triggering signal that causes the contactor to disengage the power line. In modern contactors, a microprocessor or other logic control device may be included, for coordinating the operation of contactors in related circuits.
It is generally undesirable to have the contactor trip circuits be so sensitive as to trip immediately upon the current reaching a threshold as detected by a change of state on the output of the threshold responsive current sensing element, i.e., at the least pulse on the trip signal. Such a pulse may be an anomaly such as a noise spike, that does not represent a real fault condition. Trip circuits are advantageously designed to trigger more slowly than they might, in order to reduce the incidence of nuisance trips due to such anomalies. The threshold responsive sensing means associated with the contactor(s), for example a current threshold sensor, is therefore arranged with certain timing considerations. A direct short circuit as detected by sensing load current over a high current threshold may generate a trip relatively more promptly, and a lower current threshold that remains for a predetermined time may indicate a thermal overload and can also generate a trip. However, even for a relatively fast high current threshold device, a current level over the predetermined threshold level subsists for some minimum time as the state of the sensing device switches, the necessary drive signal is generated and the contacts are physically separated to break the circuit.
During the time between the onset of a current fault and the breaking of the circuit, there is a potential for damage to the contactor apparatus, to the load device and/or to the power distribution lines, for example due to resistive heating and due to arcing. Once established, a high current level also causes an inductive voltage surge as the protective contacts open, which may cause arcing across the contacts. In an extreme case, the contacts can weld together due to arcing, necessitating a trip at a point more proximal to the power source. Arcing erodes the contacts, and over a number of successive trips, the point at which the contacts abut one another may be changed by this erosion, affecting the mechanical operation of the contactor. This problem is acute in high power distribution systems.
In the event of a direct short circuit on the load side of a contactor, there is very little parallel resistance between the power line conductors. It is possible to reduce the level of short circuit current by placing a resistance in series with the contacts and the load. Whereas the resistance of the load is many times higher than the short circuit resistance of the conductors, the current limiting series resistance can be small, and does not dissipate a great deal of power or substantially affect the load. When the load is nominally operational, most of the line voltage is across the load. When the load resistance is shunted by a short circuit, most of the line voltage is across the current limiting series resistor.
Use of a current limiting series resistor thus reduces the short circuit current and protects the contacts of the contactor as they are opened during a fault. Although the series resistance is small, there is a voltage drop across the series resistance and a dissipation of energy in the form of heat. Of course when a fault occurs and the current level increases, there is a corresponding increase in the heat dissipated by the current limiting series resistor. This heat is a problem in itself, and may cause damage to the contactor or, in the event the contactor includes solid state elements or logic devices, may produce thermal runaway that causes switching elements and logic elements to continue to conduct when they should turn off.
According to the present invention, a protective series resistance is disposed in a housing external to the contactor housing, and includes a positive temperature coefficient resistance such as a nichrome strip. Nichrome material is known for resistance heaters. The resistance of the element is minimal when cool. When heated, particularly to a red hot condition, the resistance increases substantially. Whereas the protective resistance is disposed in an external housing defining a resistance furnace or lamp that is thermally isolated from the contactor housing, the heat dissipated therein is kept away from the contactor and its control logic, improving the effectiveness and reliability of the contactor device while protecting the load and associated conductors from damage.